Editing Sun (section)

== Solar radiation ==
{{Main|Sunlight|Solar irradiance}}
[[File:Sun in fog in Lysekil.jpg|right|thumb|The Sun seen through a light fog|alt=A photograph of the sun with a layer of fog visible in front of it.]]
The Sun emits light across the [[visible spectrum]], so its colour is [[white]], with a [[CIE 1931 color space|CIE]] colour-space index near (0.3, 0.3), when viewed from space or when the Sun is high in the sky. The Solar radiance per wavelength peaks in the green portion of the spectrum when viewed from space.<ref>{{cite news |title=What Color is the Sun? |work=Universe Today |url=http://www.universetoday.com/18689/color-of-the-sun/ |url-status=live |access-date=23 May 2016 |archive-url=https://web.archive.org/web/20160525215525/http://www.universetoday.com/18689/color-of-the-sun/ |archive-date=25 May 2016}}</ref><ref>{{cite web |title=What Color is the Sun? |url=http://solar-center.stanford.edu/SID/activities/GreenSun.html |url-status=live |archive-url=https://web.archive.org/web/20171030154449/http://solar-center.stanford.edu/SID/activities/GreenSun.html |archive-date=30 October 2017 |access-date=23 May 2016 |publisher=[[Stanford University|Stanford]] Solar Center}}</ref> When the Sun is very low in the sky, [[Diffuse sky radiation|atmospheric scattering]] renders the Sun yellow, red, orange, or magenta, and in rare occasions even [[Green flash|green or blue]]. Some cultures mentally picture the Sun as yellow and some even red; the cultural reasons for this are debated.<ref name="yellow sun paradox">{{Cite journal |last=Wilk |first=S. R. |date=2009 |title=The Yellow Sun Paradox |url=http://www.osa-opn.org/Content/ViewFile.aspx?id=11147 |url-status=dead |journal=[[Optics & Photonics News]] |pages=12–13 |archive-url=https://web.archive.org/web/20120618183229/http://www.osa-opn.org/Content/ViewFile.aspx?id=11147 |archive-date=18 June 2012}}</ref> The Sun is classed as a ''G2'' star,<ref name="Phillips1995-47" /> meaning it is a [[G-type star]], with ''2'' indicating its [[effective temperature|surface temperature]] is in the second range of the G class.

The [[solar constant]] is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately {{val|1368|u=W/m2|fmt=commas}} (watts per square metre) at a distance of one [[astronomical unit]] (AU) from the Sun (that is, at or near Earth's orbit).<ref name="TSI">{{cite web |title=Construction of a Composite Total Solar Irradiance (TSI) Time Series from 1978 to present |url=http://www.pmodwrc.ch/pmod.php?topic=tsi/composite/SolarConstant |date=24 May 2006 |website=pmodwrc |url-status=dead |archive-url=https://web.archive.org/web/20110801183920/http://www.pmodwrc.ch/pmod.php?topic=tsi%2Fcomposite%2FSolarConstant |archive-date=1 August 2011 |access-date=5 October 2005}}</ref> Sunlight on the surface of Earth is [[attenuation (electromagnetic radiation)|attenuated]] by [[Atmosphere of Earth|Earth's atmosphere]], so that less power arrives at the surface (closer to {{val|1000|u=W/m2|fmt=commas}}) in clear conditions when the Sun is near the [[zenith]].<ref name="El-Sharkawi2005">{{Cite book |last=El-Sharkawi |first=Mohamed A. |title=Electric energy |date=2005 |publisher=CRC Press |isbn=978-0-8493-3078-0 |url=https://books.google.com/books?id=UokcachsYcYC&pg=PA87 |pages=87–88}}</ref> Sunlight at the top of Earth's atmosphere is composed (by total energy) of about 50% infrared light, 40% visible light, and 10% ultraviolet light.<ref name="Solar radiation">{{cite encyclopedia |entry=Radiation (Solar) |encyclopedia=Encyclopedia of Atmospheric Sciences |year=2003 |last=Fu |first=Qiang |title=Radiation (SOLAR) |editor-last1=Curry |editor-first1=Judith A. |editor-last2=Pyle |editor-first2=John A. |publisher=Elsevier |pages=1859–1863 |doi=10.1016/B0-12-227090-8/00334-1 |isbn=978-0-12-227090-1 |url=http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf |url-status=dead |archive-url=https://web.archive.org/web/20121101070344/http://curry.eas.gatech.edu/Courses/6140/ency/Chapter3/Ency_Atmos/Radiation_Solar.pdf |archive-date=1 November 2012 |access-date=29 December 2012}}</ref> The atmosphere filters out over 70% of solar ultraviolet, especially at the shorter wavelengths.<ref>{{cite web |title=Reference Solar Spectral Irradiance: Air Mass 1.5 |url=http://rredc.nrel.gov/solar/spectra/am1.5/ |website=NREL |url-status=live |archive-url=https://web.archive.org/web/20190512190812/https://rredc.nrel.gov/solar//spectra/am1.5/ |archive-date=12 May 2019 |access-date=12 November 2009}}</ref> Solar [[ultraviolet radiation]] ionises Earth's dayside upper atmosphere, creating the electrically conducting [[ionosphere]].<ref name="Phillips1995">{{Cite book |last=Phillips |first=K. J. H. |title=Guide to the Sun |url=https://books.google.com/books?id=idwBChjVP0gC&pg=PA14 |date=1995 |publisher=[[Cambridge University Press]] |isbn=978-0-521-39788-9 |pages=14–15, 34–38}}</ref>

[[Ultraviolet]] light from the Sun has [[antiseptic]] properties and can be used to sanitise tools and water. This radiation causes [[sunburn]], and has other biological effects such as the production of [[vitamin D]] and [[sun tanning]]. It is the main cause of [[skin cancer]]. Ultraviolet light is strongly attenuated by Earth's [[ozone layer]], so that the amount of UV varies greatly with [[latitude]] and has been partially responsible for many biological adaptations, including variations in [[human skin colour]].<ref>{{Cite journal |last=Barsh |first=G. S. |date=2003 |title=What Controls Variation in Human Skin Color? |journal=[[PLOS Biology]] |volume=1 |issue=1 |page=e7 |doi=10.1371/journal.pbio.0000027 |pmc=212702 |pmid=14551921 |doi-access=free}}</ref>

High-energy [[gamma ray]] [[photon]]s initially released with fusion reactions in the core are almost immediately absorbed by the solar plasma of the radiative zone, usually after travelling only a few millimetres. Re-emission happens in a random direction and usually at slightly lower energy. With this sequence of emissions and absorptions, it takes a long time for radiation to reach the Sun's surface. Estimates of the photon travel time range between 10,000 and 170,000&nbsp;years.<ref name="NASA">{{cite web |date=2007 |title=Ancient sunlight |url=http://sunearthday.nasa.gov/2007/locations/ttt_sunlight.php |url-status=dead |archive-url=https://web.archive.org/web/20090515085541/http://sunearthday.nasa.gov/2007/locations/ttt_sunlight.php |archive-date=15 May 2009 |access-date=24 June 2009 |website=Technology Through Time |publisher=NASA |issue=50}}</ref> In contrast, it takes only 2.3&nbsp;seconds for [[neutrino]]s, which account for about 2% of the total energy production of the Sun, to reach the surface. Because energy transport in the Sun is a process that involves photons in [[Thermodynamics|thermodynamic]] equilibrium with [[matter]], the time scale of energy transport in the Sun is longer, on the order of 30,000,000&nbsp;years. This is the time it would take the Sun to return to a stable state if the rate of energy generation in its core were suddenly changed.<ref>{{Cite journal |last=Stix |first=M. |date=2003 |title=On the time scale of energy transport in the sun |journal=[[Solar Physics (journal)|Solar Physics]] |volume=212 |issue=1 |pages=3–6 |bibcode=2003SoPh..212....3S |doi=10.1023/A:1022952621810 |s2cid=118656812}}</ref>

[[Electron neutrino]]s are released by fusion reactions in the core, but, unlike photons, they rarely interact with matter, so almost all are able to escape the Sun immediately. However, measurements of the number of these neutrinos produced in the Sun are [[Solar neutrino problem|lower than theories predict]] by a factor of 3. In 2001, the discovery of [[neutrino oscillation]] resolved the discrepancy: the Sun emits the number of electron neutrinos predicted by the theory, but neutrino detectors were missing {{frac|2|3}} of them because the neutrinos had changed [[flavor (particle physics)|flavor]] by the time they were detected.<ref name="Schlattl">{{Cite journal |last=Schlattl |first=H. |date=2001 |title=Three-flavor oscillation solutions for the solar neutrino problem |journal=[[Physical Review D]] |volume=64 |issue=1 |page=013009 |arxiv=hep-ph/0102063 |bibcode=2001PhRvD..64a3009S |doi=10.1103/PhysRevD.64.013009 |s2cid=117848623}}</ref>